Evolution of the Bicep Design
Written by: Forrest Pino
Approved by: Carolina Barrera
The bicep – also known as the bicep branchii, is a muscle that lies in the upper arm between the shoulder and the elbow. It’s is a structural part that offers sturdiness and strength to the overall upper-arm system. The overall arm works as a lever where the weight of the forearm and the hand are the load, the elbow is the axis or fulcrum and finally the bicep muscle when flexed is the force that will executing a work.
The following is a development write-up of the bicep design done by the manufacturing engineer in the Prosthetic Arm team, Forest Pino.
The first design concept arose from simple measurements and initial brainstorming. I measured the length of my arm and tried to develop a design that would be an adequate length and shape.
The concave nature of the lower section of the bicep was developed after studying other prosthetic arm projects with similar features. The concave feature allows for the forearm to rotate about the semi-circle at the bottom of the design.
A perforated design was conceptualized due to its ability to maintain structural integrity while reducing weight. The prosthetic arm and its various sections are limited by weight so any gaps or features that can reduce the use of plastic will help with weight and cost of the project.
Build Upon Design 1
Figure 1 – Lateral structure. First iteration of bicep
Triangular and trapezoidal features were added for strength and stability. The initial design did not contain any features that would helping with housing components. At the time, parts were not realized and the design served more as a stepping stone and base for further work.
Figure 2 – Design 1 Pre-Motor Mount
The cuts on the outer perimeter of the design were developed for braces that would be attaching and providing for structural integrity.
Figure 3 – Pre-motor Design Lateral Side
These trapezoidal cuts were made as a locking mechanism that would help pieces maintain position and provide for structural integrity. The largest side of the trapezoidal cut was planned to be the outer face of the bicep. This feature would allow interlocking piece to slide in only one direction.
Figure 4
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Figure 6
Once a possible motor was selected, an area for mounting began to take shape. The extruded circular cut in the lower half of the bicep was planned to be the area where the motor would be mounted. The shaft of the motor would protrude outward toward the outer face of the bicep. In this design, the arm that would be developed would be on the right and where the gears would line on the outer side of the arm. Originally, the twos gears were thought to be interconnected by a pulley or chain. The possible sizes of the gears were not realized at this point and a cut made for the mounting of the forearm to the bicep could not be produced yet. Having gears on the inside of the bicep or residing between the arm and the body would not be feasible due to the size of the motor and possible interference. The gears were chosen to reside on the outer most part of the arm so that interference from the user would be reduced
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Figure 8
The first assembly provided a base knowledge into how lightweight the initial design work was and how adding more plastic for the mounting of components may be necessary. The areas where the components may reside were exposed to the surrounding environment and a little more protection seemed to be necessary. The bars of plastic did not provide much confidence for stability and were eventually scrapped for ideas that utilized plastic more effectively.
Slightly More Progression
After some deliberation, it was found that the previous height of our design was too tall. We wanted to follow a design that more closely represented the size of a prosthetic for an individual with an above elbow amputation. Previously, I had taken measurements from my shoulder down to my elbow. I aimed to shrink the design while having the appropriate space for the components as well as to not construct a bulky, uncomfortable design.
Figure 9
After some research on gear availability, it become known that we had one option for purchasing gears or we would be developing a printable gear system. Originally, we visualized that the gears we would utilize would be connected by a chain or pulley but that was not necessarily the case.
The gears that were idealized for the system were larger than we had anticipated and a pulley or chain was not needed. The center-to-center distance of the two gears was more than previously thought so the design was adjusted in order to accommodate and support teeth-to-teeth interaction for the purchased gears.
Figure 10
Interlocking pieces with bored holes for fastening were the major additions to the design. They were added to improve structural integrity.
Figure 11
Sections capable of inserts were added for structural stability. Pieces were constructed in a way that would allow them to slide into the sides of the bicep. Pins or screws were to be used to fasten the brace and side together. Initially, I thought the slots were beneficial for the design but they were not necessarily capable in real world applications. The printer that is available to us may not be able to handle the bridging that occurs here. The original size of the slots were 0.1 inches (2.54 mm) and the printer does not provide infill to a design unless the print is at least 4 mm thick. This meant that the braces that would be printed for this design would only have a shell with minimal plastic making up the interior.
Figure 12
The area in which the motor would be mounted was raised compared to previous designs. This was done to ensure that the motor would be mounted to an area that would be structural sound and capable of supporting a suspended motor.
Figure 13
Trapezoidal cuts with raised planks to add addition support. The two interlocking pieces can be fastened together through the matching and aligned holes.
Figure 14
A shelf feature was added to the inner side of the wall of the bicep. The purpose of the shelf was to house or suspend the PCB above the motor. Allowing the PCB to be suspend inside the bicep gives the PCB less opportunity to overheat.
First Attempt at Gear Design
Figure 15
The first attempt at creating and mating two gears was not successful. I tried to utilize the premade gear in SolidWorks while also trying to apply the same properties to the larger gear. The gears did not provide a smooth motion and their movements interfered with one another.
Areas of Concern
Figure 16
Some troubling aspects of the design surfaced once real-world movements were considered. The degrees of freedom that were chosen for the operation were not possible with this design. This meant that alternations were needed in order for the bicep to not interfere with the forearm movement.
Newest Design
Figure 17
The newest design made it capable of providing the full degrees of freedom and thicknesses of vital housing areas were increased for adding stability.
Figure 18
Depending on the battery that is utilized, the battery may extend outward past the brace on the back of the bicep design. A transparent or detachable covering by be placed over the opening so that the battery will be covered and the customer may have access.
Figure 19
The top sectional area will house the battery we decide to use and the spacing that was created will allow for it to hold any of the batteries we have selected. The triangular gaps in the battery shelf will be utilized with Velcro to house the selected battery. The top most brace was left with a perforated areas due to our planning in the mounting phase. It is unclear on how the bicep will be connected to the mount but the untouched surface leaves us with possibilities. Holes may be added in later designs before printing or may surface afterwards with the use of a drill. The thickness of any part of the design was constructed to be at least 4 millimeters (0.1575 in) due to the printer’s inability to provide infill under that thickness.
The mass analysis of the bicep structure with the material set to ABS plastic is 1.23 lbs. with a volume of 33.41 cubic inches.
By using the estimated volume and the density properties found from a provide data sheet, another mass estimate can be determined.
PLA – density properties from NatureWorks LLC datasheet
- PLA plastic density – 1.24 g/cc
Gears were designed to resemble the gears that were available for purchase. The properties of the gears that were available for purchase were on the company website and these aspects were utilized in equation driven properties in SolidWorks.
The angle between the plane that resides on the inside of the forearm and the top brace is 59.72 degrees when the arm is fully extended backwards.
The angle between the plane that resides on the inside of the forearm and the top brace is 60.72 degrees when the arm is fully extended forward. The measurements show the full degrees of freedom being 180 degrees for the latest design.